President Trump’s new strategy for the war in Afghanistan, which he laid out Monday night in a televised address, is intended to give a badly needed boost to the campaign to push back the Taliban, step up the fight against terrorism and reverse the steady deterioration of security that has allowed devastating bombings to shake Kabul.

The strategy, which would require several thousand more troops to implement, will likely help, current and former United States commanders said. It would allow American officers to more closely advise Afghan brigades, train more Afghan special operations forces and call in American firepower.

But even those who support Mr. Trump’s strategy cast his decision as the start of yet another challenging chapter that might, at best, enable Afghan forces to regain momentum on the battlefield over the next several years, not a quick fix for the problems that have bedeviled the region for nearly 16 years.

“I do not think many believe there could be an outright victory,” said Michael O’Hanlon of the Brookings Institution, who has been an advocate of sending more troops to Afghanistan. “But if President Trump can reverse the momentum, then he could arguably claim bragging rights and achieve at least a partial strategic success.”

Mr. Trump had little room to maneuver, given the situation on the ground and the political calculus. No president was going to suggest to a war-weary American public the sort of troop surge that brought force levels to 100,000 in 2010, the peak of the American military’s presence in Afghanistan. But Mr. Trump has also concluded, as do most national security experts, that departing altogether would create a breeding ground for terrorists that threatens the West.

The president, who campaigned on pulling back on American engagement abroad, settled on his strategy as a necessary step to reverse gains made by the Taliban and to fight terrorism, including the branch in Afghanistan of the Islamic State, also known as ISIS.

“A hasty withdrawal would create a vacuum for terrorists, including ISIS and Al Qaeda,” Mr. Trump said Monday night in front of about 2,000 service members in Fort Myer, Va., near Washington.

But whether the battlefield advances projected under Mr. Trump’s plan will endure remains an open question. Sustaining the gains would require greater cooperation from Pakistan, which has allowed the Taliban and extremist groups to maintain sanctuaries on its territory, and a more determined effort by Afghan authorities in fighting corruption and improving governance.

The Trump administration acknowledged as much on Monday by casting the new troop deployments, expected to be an increase of as many as 4,000, as part of a new policy for the South Asia region, rather than Afghanistan alone. But carrying out that strategy will require deft diplomatic management from a team that is still untested in managing national security crises.

Sending more troops, however, is a big part of what American military generals have long argued is needed to help stem losses in Afghanistan. The United States has 8,400 troops allocated to the Resolute Support mission to train and advise Afghan forces, far fewer than the 13,600 American troops that retired Gen. John R. Allen, the Afghan commander from 2011 to 2013, sought from the Obama administration. Another 2,000 American forces are allocated to work with Afghan forces in counterterrorism missions.

Given such a modest footprint, the nearly 4,000 troops that Defense Secretary Jim Mattis has been authorized to add and additional reinforcements that have been solicited from NATO and allied nations make for a noteworthy increase.

“It would begin the process of arresting the difficulties the Afghans have suffered because we did not have the numbers to properly train and advise them,” General Allen said in an interview before Mr. Trump’s announcement.

In addition to the increase in troops, how they will be deployed is a major factor in whether Mr. Trump’s strategy will succeed.

Assuming Mr. Mattis sends all of the additional troops, as expected, the American military will be able to advise select Afghan brigades in the field instead of trying to mentor them from more distant headquarters. They can step up the effort to train special operations forces and, thus, substantially increase the number of Afghan commandos.

They would also enable the United States to call in air and artillery strikes on behalf of more Afghan units, making American firepower more effective.

“These are going to be people specifically designed, trained and organized and equipped to go in and advise them how you take the hill, get them the air support and artillery support and rocket support that will enable them,” Mr. Mattis told Congress in June as he laid out his proposal for the additional forces.

Even if the plan unfolds as designed, the military itself has argued that still more steps are needed.

Achieving lasting gains will depend heavily on the Trump administration’s ability to persuade Pakistan to shut down the sanctuaries that the Taliban and extremist groups like the Haqqani network enjoy on its territory, a goal that has eluded both Republican and Democratic administrations.

“No partnership can survive a country’s harboring of militants and terrorists who target U.S. service members and officials,” Mr. Trump said. “It is time for Pakistan to demonstrate its commitment to civilization, order and to peace.”

To bring Pakistan along, Trump administration officials have been discussing a mix of steps. The United States could reduce aid, slap sanctions on Pakistani officials and perhaps expand the military’s authority to conduct airstrikes on Pakistan’s side of the border.

Pakistan, however, is only part of the problem. The Russians have been talking to the Taliban and, according to American officials, may be providing material support. The Kremlin’s policy may be a hedge against the growth of the Islamic State’s influence in Afghanistan, the possible departure of American forces or simply a way to make life harder for the Americans. Whatever the Russian motivations, they add to the challenges in dealing with Moscow and in Afghanistan.

What remains unclear is whether Mr. Trump’s strategy will pressure the Taliban strongly enough to prompt them to come to the negotiating table to work out a peace settlement with the Afghan authorities. The much larger force of 100,000 troops that President Barack Obama deployed to Afghanistan did not batter the Taliban sufficiently to yield a diplomatic settlement.

But even if a successful negotiation is not possible, the Pentagon is likely to argue that stepping up the American effort in Afghanistan and buttressing the authorities in Kabul will enable the United States to maintain a platform for conducting counterterrorism operations in Afghanistan and Pakistan.

Mr. Trump often talks about the importance of winning. But as the Afghan war nears the end of its 16th year, not losing may be the more immediate and achievable objective.

A version of this article appears in print on , on Page A11 of the New York edition with the headline: U.S. Plan for Afghanistan May Help, but Few Expect ‘Outright Victory’. Order Reprints | Today’s Paper | Subscribe